The invention relates to a power control device and method for calibrating the power of a transmitter or receiver of a mobile communication network in which burst signals are applied to the transmitter. The burst signals include a fixed training sequence.
More generally, the invention relates to Power Control, in particular Digital Power Control of a transmitter such as a Transceiver (TRX, e.g. Transceiver Card) or a BTS (Base Transceiver Station) Transmitter. More specifically, the invention aims at providing a power control function applicable to a device, system, and method requiring power calibration.
In existing GSM compatible basestations, Power Control is implemented by a combination of two methods: closed loop, when the transmitter is on, and open loop when the transmitter is off. The closed loop mode is a real time control in which the output power is sampled and compared to a reference signal, and the gain is corrected. The closed loop part of the power control operation is also used to control the ramp shape between timeslots so that the power versus time mask is met.
The open loop part of the power control function is used in the off period between timeslots as the linear range of the output detector limits the closed loop operation.
In the standard GSM system, with a constant RF (Radio Frequency) envelope, power control can be easily carried out utilizing an integrator as a loop filter. If there is any ripple on the envelope, the integrator, depending upon the loop bandwidth, attempts to recover the variation on the envelope by altering the attenuation on the RF path. However, the analog (closed loop) power control schemes as used with GMSK (Gaussian Minimum Shift Keying), have difficulty with EDGE (Enhanced Data for GSM Evolution) 8PSK modulation, as the signal has no longer a constant envelope.
The analog power control has been enhanced for EDGE by adding a replica of the modulation envelope onto the power control voltage and comparing it to the detected voltage. Switching the loop bandwidth during the burst enables this scheme to meet the ramping requirements of GSM, make the power control scheme less sensitive to delay and minimise amplitude error.
As mentioned above, the analog scheme requires a combination of closed loop and open loop control as the linear range of the detector is not sufficient for the Tx Off state (Tx=transmitter). Such as solution is difficult to implement and often causes distortion of the signal due to switching transients caused by the switching of the loop bandwidth. Transients may also occur when switching between closed loop and open loop operation as the detector range is not sufficient to detect the whole envelope. Additionally, as the power ramping is controlled by the closed loop, overshoot of the ramp often occurs, giving type approval difficulties.
Further, when building a smart antenna (SA) BTS utilising beam steering the relative losses and phase lengths between the N columns of the array are critical to performance. These relative differences are not constant over time, temperature and frequency. The phase length and losses need to be measured in a non-obtrusive fashion to meet type approval requirements. The measurement and adjustment of these differences needs to be automatic.
In the past, beam steering systems have tended to use individual RF line-ups for each transmit path, with the calibration being performed in the baseband processing, which has resulted in very expensive systems.